Portable devices having wireless communications capabilities are currently available in several different forms, including mobile telephones, personal digital assistants and hand held scanners.
The demand for wireless connectivity from portable devices is rapidly expanding. As a result, the demand for high performance, low cost, and cosmetically appealing antenna systems for such devices is also increasing.
One type of antenna commonly used in portable wireless devices is the monopole whip. A monopole whip antenna is essentially a wire that extends along or away from the device and is fed by the printed circuit board (PCB) of the device. One problem of this unbalanced design is that radio frequencies (RF) currents induced on the PCB may cause receiver desensitization, thereby limiting the useful range of the device.
In a monopole whip design as described above, and other unbalanced designs used in similar applications, the PCB may function as a part of the antenna. As a result, the PCB may also radiate a portion of a signal being transmitted, causing operating characteristics of the antenna such as gain, radiation pattern, and driving point impedance to become dependent on qualities of the PCB such as size, shape, and proximity to other structures (such as a display, a cable, a battery pack, etc.). Therefore, it may become necessary to redesign the antenna to achieve a similar performance with different applications and/or different types of devices.
Radiation by a PCB due to RF coupling with an unbalanced antenna may also cause efficiency losses. In a mobile phone application, for example, radiation of a PCB that is placed next to the user's head may be wasted due to absorption of the radiating fields by the user's head and hand. In addition to reducing the efficiency of the device, this effect may also increase the specific absorption rate (SAR) beyond regulatory limits.
A coaxial sleeve dipole is a balanced antenna that tends to de-couple the antenna system from the PCB or device to which it is connected. Such an antenna is constructed of coaxial cable, where the center conductor extends beyond the outer conductor, and the outer conductor is rolled back to form a jacket. One advantage of this design is that if the jacket has the right length, then current which otherwise might distort the radiation pattern may be impeded from flowing along the outer surface of the feed cable. Unfortunately, coaxial sleeve dipoles are too bulky and heavy to be practical for use in small portable devices and are not compatible with the small, slim profiles of present portable wireless devices. Additionally, coaxial sleeve dipoles are relatively expensive.
Accordingly, it is an object of the present application to obviate or mitigate the above disadvantages.